Only rehash if required
Allow cpanm to be installed through proxy

Curl wasn't working correctly while under a proxy, was getting this error:
curl: (35) error:14077458:SSL routines:SSL23_GET_SERVER_HELLO:reason(1112)
Merge pull request #70 from dave-miles/fix/cpanm-shim

Only rehash cpanm if required
Merge pull request #69 from stephenap07/master

Allow cpanm to be installed through proxy