Use gzencode instead of gzcompress

```gzencode``` produces a gzip compatible file format. ```gzcompress``` produces something slightly different.

See here for further details:  http://stackoverflow.com/questions/22950285/how-output-data-with-gzip-header-and-not-deflate-header
Merge branch 'patch-1' of https://github.com/tommygnr/AsseticStaticGzipBundle into tommygnr-patch-1
Merge branch 'tommygnr-patch-1'