Badges are crisper as svg

[skip ci]
Merge pull request #827 from rspec/badges_as_s

Switch readme badges to svg
Validate invocation args for verified null object doubles.
Merge pull request #829 from rspec/fix-null-object-verified-double-arg-verification

Validate invocation args for verified null object doubles.
Add a `description` to `receive` matcher.
Add a description for `receive_message_chain` matcher.
Add a description for `receive_messages` matcher.
Add changelog entry.
Merge pull request #828 from rspec/add-description-for-receive-matcher

Add description for matchers
allow verifying doubles to use custom names in exceptions
add class documentation about named verifying doubles
mention naming verifying doubles in README
Merge pull request #826 from e2/issue_825_named_verifying_doubles

allow naming verifying doubles just like ordinary doubles
Add changelog entry for #826.

[ci skip]
Make `RSpec::Mocks::Constant.original(name)` handle invalid names gracefully.

Since this API returns an object that provides lots of details
about the constant (rather than just the original value), it's
not meant to raise errors when given constant names that can’t
resolve or are invalid -- instead the returned object indicates
that the provided name was invalid.
Merge pull request #833 from rspec/fix-constant-original-for-bad-const-names

Make `RSpec::Mocks::Constant.original(name)` handle invalid names gracefully.
Updated travis build scripts (from rspec-dev)
Merge pull request #834 from rspec/update-travis-build-scripts-2014-12-08-for-master

Updates from rspec-dev (2014-12-08)
Revert "Updated travis build scripts (from rspec-dev)"

This reverts commit addf8949b9e263fe2dcd526ec4715a736fdf9b7d.

[ci skip]
Updated travis build scripts (from rspec-dev)