Allow stubbing method to which any instance responds.

Fixes #200.

Previously we were not calling `Object#respond_to?` on an instance of the
class. By using `Object#allocate` we can instantiate an example instance of
the class so we can interrogate it using `Object#respond_to?`.

Obviously this is not perfect, given that a method might be defined in the time
between stubbing and invocation, but it means that both
`Mocha::ObjectMethods#method_exists?` and `Mocha:ClassMethods#method_exists?`
now work in a similar way.
No need to update rubygems on Travis CI any more.

This `before_install` block was added in [1], because of a bug in the version
of Rubygems provided by Travis CI by default (v2.2.0). We need a fix which is in
Rubygems v2.2.1 and the default version provided by Travis CI is now v2.2.2.

[1] 3128fb8475bf9af81532f7a71b0fe6ca376f8c6e