Drop support for Python 2.5

[hugovk]

Closes #47.

[hugovk]

Here's the pip installs for pyyaml from PyPI for the last month -- nothing for Python 2.5:

$ pypinfo --percent --pip pyyaml pyversion
python_version percent download_count
-------------- ------- --------------
2.7              89.4%     11,732,683
3.5               3.6%        470,139
3.6               3.3%        430,133
3.4               2.7%        351,614
2.6               1.0%        127,184
3.3               0.1%          7,931
3.7               0.0%          3,638
3.2               0.0%            284
None              0.0%            126
3.1               0.0%              2
2.8               0.0%              2

[hugovk]

Updated to resolve conflicts.

[perlpunk]

Thanks! It seems everything in this PR was patched by other PRs. Closing

[hugovk]

Great! I think #47 can be closed now.