should be not important. In particular, when invsigma was <10-12
sigma was set as equal to invsigma. Now it is set to zero,
which I think is the expected behavior.

When conda build fails, the travis job should return an error.

@tonigi

I verified it, I will then modified a regtest so to keep checking for this

python module should always be called `plumed`, irrespectively of
prefix/suffix. Notice that if installing multiple plumed versions
side by side different python modules will anyway end up in different
directories (`lib/program_name`).