Fix Ruby 1.9 build (#680)
Fix Ruby 1.9 build (#680)

Lock a dependency on an older version so that it does not raise a syntax
error.

```
An error occurred while loading spec_helper.
Failure/Error: require "webmock/rspec"

SyntaxError:
  /home/semaphore/appsignal-ruby/.bundle/gems/rexml-3.2.4/lib/rexml/xpath.rb:34: syntax error, unexpected tPOW, expecting ')'
        parser = XPathParser.new(**options)
                                   ^
  /home/semaphore/appsignal-ruby/.bundle/gems/rexml-3.2.4/lib/rexml/xpath.rb:63: syntax error, unexpected tPOW, expecting ')'
        parser = XPathParser.new(**options)
                                   ^
  /home/semaphore/appsignal-ruby/.bundle/gems/rexml-3.2.4/lib/rexml/xpath.rb:73: syntax error, unexpected tPOW, expecting ')'
        parser = XPathParser.new(**options)
                                   ^
# ./.bundle/gems/rexml-3.2.4/lib/rexml/element.rb:6:in `require_relative'
# ./.bundle/gems/rexml-3.2.4/lib/rexml/element.rb:6:in `<top (required)>'
# ./.bundle/gems/rexml-3.2.4/lib/rexml/document.rb:3:in `require_relative'
# ./.bundle/gems/rexml-3.2.4/lib/rexml/document.rb:3:in `<top (required)>'
# ./.bundle/gems/crack-0.4.5/lib/crack/xml.rb:5:in `require'
# ./.bundle/gems/crack-0.4.5/lib/crack/xml.rb:5:in `<top (required)>'
# ./.bundle/gems/webmock-2.3.2/lib/webmock.rb:5:in `require'
# ./.bundle/gems/webmock-2.3.2/lib/webmock.rb:5:in `<top (required)>'
# ./.bundle/gems/webmock-2.3.2/lib/webmock/rspec.rb:1:in `require'
# ./.bundle/gems/webmock-2.3.2/lib/webmock/rspec.rb:1:in `<top (required)>'
# ./spec/spec_helper.rb:13:in `require'
# ./spec/spec_helper.rb:13:in `<top (required)>'
```
appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal-ruby