Using gzip until Docker image gets updated

As per voutcn/megahit#360 pigz is not currently installed in the MEGAHIT biocontainer images (singularity: https://depot.galaxyproject.org/singularity/megahit:1.2.9--h5b5514e_2 - docker: quay.io/biocontainers/megahit:1.2.9--h5b5514e_3). Therefore, the NF-Core module will fail to compress the contigs using pigz. 

This commit resolves the described fix.

modules/nf-core/megahit/main.nf

@@ -33,10 +33,7 @@ process MEGAHIT {
             $args \\
             --out-prefix $prefix
 
-        pigz \\
-            --no-name \\
-            -p $task.cpus \\
-            $args2 \\
+        gzip \\
             megahit_out/*.fa \\
             megahit_out/intermediate_contigs/*.fa
 
@@ -54,10 +51,7 @@ process MEGAHIT {
             $args \\
             --out-prefix $prefix
 
-        pigz \\
-            --no-name \\
-            -p $task.cpus \\
-            $args2 \\
+        gzip \\
             megahit_out/*.fa \\
             megahit_out/intermediate_contigs/*.fa