Move sourcing rvm after installation of rvm.
Respect http_proxy variable of host for download within the VM.

This should help using the box in environments with proxy-only internet,
like many companies have and speeds up repeated VM provisioning with
a caching proxy.
 Use 'make -jX' where possible to increase the speed of VM provisioning.
Initialize .bash_profile so that grep does not complain about it's missing.
Use a extraced directory of tar instead of the downloaded file to trigger
setup of mingw-w64 compilers.

This should decrease issues with broken downloads.