feat: make semantic-release CI agnostic

- Remove `@semantic-release/condition-travis` from the default plugins
- Verify the current branch in the core
- Verify the build is not triggered by a PR in the core
- Run in dry-run mode if not triggered on CI
- Dry-run mode runs the `verifyConditions` plugins, allowing to detect configuration error locally
- Return without error when no version has to be released due to no changes
- Return without error if the build is triggered from a PR
- Return without error if the current branch is not the configured branch
- CLI return with exit code 1 if there is a `semanticReleaseError`, allowing to fail builds in case of config error, missing token etc...

BREAKING CHANGE: `semantic-release` doesn't make sure it runs only on one Travis job anymore.
The CI configuration has to be done such that `semantic-release`
- runs only once per build
- runs only after all tests are successful on every jobs of the build
- runs on Node >=8

This can easily be done with [travis-deploy-once](https://github.com/semantic-release/travis-deploy-once).

Migration Guide

Modify your `.travis.yml` to use `travis-deploy-once`.
Replace:
```yaml
after_success:
  - npm run semantic-release
```
by:
Replace
```yaml
after_success:
  - npm install -g travis-deploy-once@4
  - travis-deploy-once "npm run semantic-release"
```

cli.js

@@ -48,12 +48,10 @@ module.exports = async () => {
       await require('.')(pickBy(program.opts(), value => !isUndefined(value)));
     }
   } catch (err) {
-    // If error is a SemanticReleaseError then it's an expected exception case (no release to be done, running on a PR etc..) and the cli will return with 0
-    // Otherwise it's an unexpected error (configuration issue, code issue, plugin issue etc...) and the cli will return 1
+    process.exitCode = 1;
     if (err.semanticRelease) {
       logger.log(`%s ${err.message}`, err.code);
     } else {
-      process.exitCode = 1;
       logger.error('An error occurred while running semantic-release: %O', err);
     }
   }