fix(travis): try using a subshell for cargo cmd
fix(travis): try using a subshell for cargo cmd

Apparently travis doesn't execute cargo in the right sub-directory.
Might be a difference in the way make works

Related to #8

steveklabnik/google-apis-rs,steveklabnik/google-apis-rs,steveklabnik/google-apis-rs,steveklabnik/google-apis-rs