Pin coverage module at 4.5.4

This is a mitigation for nedbat/coveragepy#916

.travis.yml

@@ -12,7 +12,7 @@ python:
     - 'pypy3'
 install:
     - pip wheel -f wheelhouse cython pysam numpy || true
-    - pip install -f wheelhouse cython pysam pyfasta coverage pyvcf numpy || true
+    - pip install -f wheelhouse cython pysam pyfasta 'coverage==4.5.4' pyvcf numpy || true
     - pip install -f wheelhouse -e git+https://github.com/biopython/biopython.git#egg=biopython || true
     - python setup.py install
     - if [ ! -f samtools-1.2 ]; then curl -sL https://github.com/samtools/samtools/releases/download/1.2/samtools-1.2.tar.bz2 | tar -xjv; fi