Unpin development requirements

Minimum required version will be updated if a specific feature or a
bug fix from a newer version is required.

Author: jcfr

requirements-dev.txt

@@ -1,9 +1,9 @@
-codecov==2.0.5
-coverage==4.2
-flake8==3.0.4
-pytest==3.0.3
-pytest-cov==2.4.0
-pytest-runner==2.9
-pytest-virtualenv==1.2.4
-scikit-build==0.4.0
-virtualenv==15.0.3
+codecov>=2.0.5
+coverage>=4.2
+flake8>=3.0.4
+pytest>=3.0.3
+pytest-cov>=2.4.0
+pytest-runner>=2.9
+pytest-virtualenv>=1.2.4
+scikit-build>=0.4.0
+virtualenv>=15.0.3