MAINT: remove pinned versions from requirements.txt

Author: jakevdp

requirements.txt

@@ -1,11 +1,11 @@
-numpy==1.11.1
-pandas==0.18.1
-scipy==0.17.1
-scikit-learn==0.17.1
-scikit-image==0.12.3
-pillow==3.4.2
-matplotlib==1.5.1
-seaborn==0.7.0
+numpy
+pandas
+scipy
+scikit-learn
+scikit-image
+pillow
+matplotlib
+seaborn
 jupyter
 notebook
 line_profiler