Allow Bokeh version 1.x.x for Python 2 compatibility

requirements.txt

@@ -3,7 +3,7 @@ scipy>=0.18.0
 pandas>=0.18.1
 matplotlib>=1.5.1
 seaborn>=0.7.1
-bokeh>=2.2.3
+bokeh>=1.0.0
 pysam>=0.9.1
 pybedtools>=0.7.8
 cutadapt>=1.10