fixed postinstall in cygwin

Error message:

> meteorite@0.6.10 postinstall C:\Users\%username%\AppData\Roaming\npm\node_modules\
meteorite
> ./completions/postinstall.sh

'.' is not recognized as an internal or external command,
operable program or batch file.
Merge pull request #177 from veell/patch-1

fixed postinstall in cygwin