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"
```

package.json

@@ -16,7 +16,6 @@
   },
   "dependencies": {
     "@semantic-release/commit-analyzer": "^5.0.0",
-    "@semantic-release/condition-travis": "^7.0.0",
     "@semantic-release/error": "^2.1.0",
     "@semantic-release/github": "^3.0.0",
     "@semantic-release/npm": "^2.0.0",
@@ -25,6 +24,7 @@
     "commander": "^2.11.0",
     "cosmiconfig": "^3.1.0",
     "debug": "^3.1.0",
+    "env-ci": "^1.0.0",
     "execa": "^0.8.0",
     "get-stream": "^3.0.0",
     "git-log-parser": "^1.2.0",