Tweaks to build stuff

requirements.txt

@@ -2,4 +2,4 @@ six>=1.10.0,<2.0
 numpy>=1.11.1,<2.0
 nibabel>=2.1,<3.0
 indexed_gzip>=0.3.3,<0.4
-wxPython>=3.0.2.0,<=4.0.0a2
+wxPython>=3.0.2.0,<4.1

setup.py

@@ -106,6 +106,7 @@ setup(
     setup_requires=['pytest-runner', 'sphinx', 'sphinx-rtd-theme', 'mock'],
 
     tests_require=['mock',
+                   'coverage',
                    'pytest-cov',
                    'pytest-html',
                    'pytest-runner',