[sailfish-browser] Fix invoker launching

Both PWD and GRE_HOME were not set to env when launching from
application grid through invoker. So, let's set GRE_HOME
over here.

Due to above XRE_GetEmbedLite() (Gecko embedlite) function was not
found when launching through invoker.
Merge pull request #89 from rainemak/fixInvokerLaunch

[sailfish-browser] Fix invoker launching