Temporarily fix pysam on 0.7.8 (0.8.0 fails on Python 3)

Before we figure out what causes this, let's have a working test suite by
fixing pysam on the latest working release.

Traceback:

    Traceback (most recent call last):
      File "/home/travis/build/jamescasbon/PyVCF/build/lib.linux-x86_64-3.3/vcf/test/test_vcf.py", line 1109, in testNoVariantsInRange
        fetched_variants = self.reader.fetch('20', 14370, 17329)
      File "/home/travis/build/jamescasbon/PyVCF/build/lib.linux-x86_64-3.3/vcf/parser.py", line 623, in fetch
        self.reader = self._tabix.fetch(chrom, start, end)
      File "ctabix.pyx", line 345, in pysam.ctabix.Tabixfile.fetch (pysam/ctabix.c:4241)
    TypeError: expected bytes, str found

See #175
Partial support for VCFv4.2

- Add R as an INFO field count (number of alleles including reference).
- Support the optional Source and Version keys on INFO metainformation.

Thanks alot @travc for contributing these fixes!

See #172
Merge pull request #174 from jamescasbon/vcf-4.2

Partial support for VCFv4.2
Restrict pysam to version 0.7.8.

pysam v.0.8.0 changed its interface such that it deals entirely with
bytes, which has downstream effects for Python 3, which uses unicode as
the native string type. In 0.8.0, the Tabixfile.fetch method now expects
to receive bytes instead of a (unicode) string for the the "reference"
argument. This is evidenced in a recent rebuild of PyVCF's "master"
since its release:

https://travis-ci.org/gotgenes/PyVCF/jobs/34783332#L1936

pysam is currently wrestling with dealing with unicode and bytes issues
in Python 3: https://github.com/pysam-developers/pysam/issues/29