Handle bowtie2 being a relative symlink

Hi, Ben.

When bowtie2 is a relative symlink, I get the following error. This patch fixes the issue.

Cheers,
Shaun

```sh
➜  /  cd /
➜  /  which bowtie2
/usr/local/bin/bowtie2
➜  /  ls -l /usr/local/bin/bowtie2
lrwxr-xr-x  1 sjackman  admin  35 Feb 16 21:11 /usr/local/bin/bowtie2 -> ../Cellar/bowtie2/2.2.0/bin/bowtie2
➜  /  bowtie2 --version
(ERR): Expected bowtie2 to be in same directory with bowtie2-align:
/Cellar/bowtie2/2.2.0/bin/
Exiting now ...
```