Fix Deployment Notification documentation

This is my attempt to correct the various inconsistent / incomplete / incorrect online documentation sources for New Relic Deployments, which confused me when I tried to use this API.

I am using the internal New Relic **How-to: track deployments with New Relic** /deployments/instructions as a master reference.

* added `revision` to the rdoc as it was missing from http://newrelic.github.io/newrelic_api/NewRelicApi/Deployment.html
* removed `environment` and `changes` from the README because they were not documented elsewhere, and didn't appear to be recognised by the API when I tested them
* removed `appname` from the README and tests, because even though it works, it isn't documented in the How To and `app_name` seems to be the preferred name as used elsewhere

Unrelated to this PR specifically, I had to do major surgery on the Gemfile + lock to get the tests to run at all, and two of the tests fail because the integration server test app is not returning any threshold values.

e.g.:
`curl -H "x-api-key:8022da2f6d143de67e056741262a054547b43479" "https://api.newrelic.com/api/v1/accounts.xml?include=application_health"`

returns:
```xml
<?xml version="1.0" encoding="UTF-8"?>
<accounts type="array">
  <account>
    <id type="integer">128523602</id>
    <name>Gold</name>
    <event-feed-uri>/account_feeds/8022da2f6d143de67e056741262a054547b43479000/events.rss</event-feed-uri>
    <applications type="array">
      <application>
        <id type="integer">449331907</id>
        <name>gold app</name>
        <overview-url>https://test.local/accounts/128523602/applications/449331907</overview-url>
        <servers-url>https://test.local/api/v1/accounts/128523602/applications/449331907/servers</servers-url>
        <threshold-values type="array">
        </threshold-values>
      </application>
    </applications>
  </account>
</accounts>
```