Pinned versions packages interpreted as float numbers

Pinned packages listed in conda_build_config.yaml have their version number interpreted as floats when in format x.x and as strings when in format x.x.x when loaded by the pyyaml library in bob.devtools/bob/devtools/build.py in the load_packages_from_conda_build_config method.

The problem is, for instance, that if we want the package python-graphviz=0.20 conda will search for python-graphviz=0.2 and may return an error as it doesn't find the package in this version.

A solution could be to write all the package versions' numbers between quotes so they are all interpreted as strings.