Rest-client >1.7.0 doesn't support ruby 1.8.7

Since version 1.7.2 rest-client doesn't support ruby 1.8.7.

Locked version of rest-client to <= 1.7.0 as newer versions require ruby > 1.9.2

I've locked it to this version as it appears to be the latest one with 1.8.7 support.