TEST: Use --ignore-installed as workaround of conda

`conda` ships with setuptools but lacks a corresponding
easy-install.pth. Attempting to upgrade setuptools using `pip` fails
because easy-install.pth is missing, despite the presentation of the
conda-originating setuptools as an easy_install egg. It is a known bug
of `conda` that is documented at the following:

pypa/pip#2751
ContinuumIO/anaconda-issues#542

.travis.yml

@@ -31,7 +31,7 @@ before_install:
   - conda install -q -y nomkl
   - conda install -q -y numpy scipy
   - conda info -a
-  - pip install --upgrade pip setuptools
+  - pip install --ignore-installed --upgrade pip setuptools
   - pip install nose
   - pip install mock