Fix "invalid byte sequence" errors in `pull-request`

If the user's `LANG` environment variable doesn't include "UTF-8", the
default external encoding in Ruby will be US-ASCII and as such, string
operations will fail dealing with pull request message or changelog that
contains non-ASCII characters.

This explicitly configures external encoding in hub to default to "UTF-8".
This applies to data over ARGV, `$stdin.read`, and reading files.

Fixes #566
Fix generating standalone script in bundle context

`bundle exec rake standalone` in development mode would try to shell out
to `ruby --disable-gems` to detect 1.9 support, but that would fail
because Bundler tries to propagate itself through RUBYOPT and
`--disable-gems` obviously doesn't play along with it.
Merge branch 'encoding-fix'