Display hashdigest instead of hash object upon failure.

I noticed that when an md5 has failure occurs, it shows the hash object reference instead of the hex digest. This is a simple fix to display the actual hex digest for visual comparison.

```
[localhost] out: Downloading/unpacking sqlalchemy==0.8.0 (from zql==0.1.6)
[localhost] out:   Downloading SQLAlchemy-0.8.0.tar.gz (3.8MB): 1.3MB downloaded
[localhost] out:   Hash of the package https://pypi.python.org/packages/source/S/SQLAlchemy/SQLAlchemy-0.8.0.tar.gz#md5=11cd07ca81fab78d53f2922b5fb187a3 (from https://pypi.python.org/simple/SQLAlchemy/) (<md5 HASH object @ 0x287d260>) doesn't match the expected hash 11cd07ca81fab78d53f2922b5fb187a3!
[localhost] out: Bad md5 hash for package https://pypi.python.org/packages/source/S/SQLAlchemy/SQLAlchemy-0.8.0.tar.gz#md5=11cd07ca81fab78d53f2922b5fb187a3 (from https://pypi.python.org/simple/SQLAlchemy/)
[localhost] out: Storing complete log in /home/josh/.pip/pip.log
```