Let aws_completer command account for LC_CTYPE of UTF-8

[quiver]

aws command had the same problem and was already fixed in #945.

How to Reproduce

$ uname -a
Darwin latok 14.1.0 Darwin Kernel Version 14.1.0: Thu Feb 26 19:26:47 PST 2015; root:xnu-2782.10.73~1/RELEASE_X86_64 x86_64
$ export LC_CTYPE=UTF-8
$ printenv LC_CTYPE
UTF-8
$ complete -C aws_completer aws
$ aws s3 <TAB>Traceback (most recent call last):
  File "/usr/local/bin/aws_completer", line 16, in <module>
    import awscli.completer
...
    raise ValueError, 'unknown locale: %s' % localename
ValueError: unknown locale: UTF-8

Writing an integration test was a bit too heavy for me(there's no integration test for aws_completer at the moment), so test case is not included in this PR.

[coveralls]

Coverage remained the same at 92.77% when pulling 7348f7c on quiver:issue-945-for-aws_completer into abbff5f on aws:develop.

[jamesls]

Yeah, I agree, integration tests for this would likely require something like pexpect or something similar. Thanks for the PR.