Allow to change swig versions compiled

The SWIG_VERSION environment variablle allows to choose which swig versions are tested. If it is not set, the default values are used.

The reason for this is that YARP tests on travis-ci are often failing because they exceed the 50 minutes time limit, therefore I suggest to reduce the number of version tested (perhaps just 1.3.40, 2.0.12, and 3.0.2?)