Isolate ruby environment and set java heap size

* Clear GEM_HOME and GEM_PATH variables to ensure we are only using
  the gems baked into the kibana jar and don't accidentally use
  the user/system's gem paths (Fixes #1531)
* Set JVM heap size to 512m. If we don't do this, the default (for the
  common JVMs) is 25% of system ram, which is not a desirable default.