use Python 2.x explicitly for node-gyp compat

[dholdren]

Description of the Change

If you have installed Python 3 and it is your default python, then installing some packages will fail. This change fixes the ambiguity by using the python2 command that is available and will always use Python 2.x

When python is version 3.x

➜  ~ apm --version
apm  1.18.1
npm  3.10.10
node 6.9.5 x64
python 3.6.1
git 2.11.0
➜  ~ apm install term3
Installing term3 to /Users/dholdren/.atom/packages ✗

> [email protected] install /private/var/folders/q4/503myxfn11z2hrmywy0zhmr00000gn/T/apm-install-dir-117612-85525-asrw3u.lx7f7ds4i/node_modules/term3/node_modules/ptyw.js
> node-gyp rebuild

/private/var/folders/q4/503myxfn11z2hrmywy0zhmr00000gn/T/apm-install-dir-117612-85525-asrw3u.lx7f7ds4i
└── (empty)


npm WARN deprecated [email protected]: react-tools is deprecated. For more information, visit https://fb.me/react-tools-deprecated
npm WARN deprecated [email protected]: graceful-fs v3.0.0 and before will fail on node releases >= v7.0. Please update to graceful-fs@^4.0.0 as soon as possible. Use 'npm ls graceful-fs' to find it in the tree.
npm WARN deprecated [email protected]: Please update to minimatch 3.0.2 or higher to avoid a RegExp DoS issue
gyp WARN download NVM_IOJS_ORG_MIRROR is deprecated and will be removed in node-gyp v4, please use IOJS_ORG_MIRROR
gyp ERR! configure error
gyp ERR! stack Error: Python executable "/Applications/Atom.app/Contents/Resources/app/apm/bin/python-interceptor.sh" is v3.6.1, which is not supported by gyp.
gyp ERR! stack You can pass the --python switch to point to Python >= v2.5.0 & < 3.0.0.
gyp ERR! stack     at failPythonVersion (/Applications/Atom.app/Contents/Resources/app/apm/node_modules/node-gyp/lib/configure.js:454:14)
gyp ERR! stack     at /Applications/Atom.app/Contents/Resources/app/apm/node_modules/node-gyp/lib/configure.js:443:9
gyp ERR! stack     at ChildProcess.exithandler (child_process.js:197:7)
gyp ERR! stack     at emitTwo (events.js:106:13)
gyp ERR! stack     at ChildProcess.emit (events.js:191:7)
gyp ERR! stack     at maybeClose (internal/child_process.js:877:16)
gyp ERR! stack     at Socket.<anonymous> (internal/child_process.js:334:11)
gyp ERR! stack     at emitOne (events.js:96:13)
gyp ERR! stack     at Socket.emit (events.js:188:7)
gyp ERR! stack     at Pipe._handle.close [as _onclose] (net.js:498:12)
gyp ERR! System Darwin 16.6.0
gyp ERR! command "/Applications/Atom.app/Contents/Resources/app/apm/bin/node" "/Applications/Atom.app/Contents/Resources/app/apm/node_modules/.bin/node-gyp" "rebuild"

When python is version 2.x

➜  ~ pyenv global system
➜  ~ python --version
Python 2.7.13
➜  ~ apm --version
apm  1.18.1
npm  3.10.10
node 6.9.5 x64
python 2.7.13
git 2.11.0
➜  ~ apm install term3
Installing term3 to /Users/dholdren/.atom/packages ✓

Benefits

This will allow installing packages that use gyp when the system Python is version 3.x

Applicable Issues

atom#698
Floobits/atom-term3#88