fix dep version to improve reproducibility

requirements.txt

 pysam==0.15.2
-matplotlib>=3.0
-pandas>=0.24
-numpy>=1.16
-pillow>=5.4
-psutil>=5.6
-scipy>=0.16.1
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+matplotlib==3.1.1
+pandas==0.25.2
+numpy==1.17.3
+pillow==6.2.0
+psutil==5.6.3
+scipy==1.3.1
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