CI: pin pytables to valid build
xref #17757
not sure what is going on at conda-forge but
```
                            3.4.2               np112py27_1  conda-forge     
                             3.4.2               np112py35_1  conda-forge     
                             3.4.2               np112py36_1  conda-forge     
                             3.4.2               np113py27_1  conda-forge     
                             3.4.2               np113py35_1  conda-forge     
                             3.4.2               np113py36_1  conda-forge     
                             3.4.2                    py27_2  conda-forge     
                             3.4.2                    py35_2  conda-forge     
                             3.4.2                    py36_2  conda-forge     
```
These latest builds, the _2 are completely bogus. They must be built against a version of numpy.
