Add tophat2/2.0.13

so according to tophat2 changelog, v2.0.13 removed SAMtools as an
external dependency in order to avoid incompatibility issues with recent
and future changes of SAMtools and its code library (an older, stable
SAMtools version is now packaged with TopHat).

Signed-off-by: James Oguya <oguyajames@gmail.com>