update Webmock to Ruby 2.0-compatible version

References #272
normalize "Accept-Encoding" for Ruby 2.0 in tests

For decompress tests, specifying "Accept-Encoding: identity" will
configure Net::HTTP in 2.0 to not auto-decompress response bodies,
giving VCR a chance to do it manually.

For other tests, it simply normalizes the header value between Ruby
versions so that the cassette content is predictable.
don't pass an Addressable::URI to Net::HTTP

It chokes. References #272
normalize response charset for Ruby 2.0 in tests

Ruby 2.0 initializes HTTP response bodies as UTF-8 (coincidentally,
Sinatra indicates by default that the responses it sends are UTF-8, but
Net::HTTP doesn't pay attention to that), where older Rubies interpreted
them as US-ASCII and thus VCR serialized it that way. For the purpose of
this test, force the response body to US-ASCII so that the outcome of
serialization is normalized between different Ruby versions.
enable checking code coverage on Ruby 2.0
don't tolerate Ruby 2.0 failures anymore in CI
Merge branch 'ruby2'

Closes #272