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"
```

lib/plugins/definitions.js

@@ -6,7 +6,7 @@ const validatePluginConfig = conf => isString(conf) || isString(conf.path) || is
 
 module.exports = {
   verifyConditions: {
-    default: ['@semantic-release/npm', '@semantic-release/github', '@semantic-release/condition-travis'],
+    default: ['@semantic-release/npm', '@semantic-release/github'],
     config: {
       validator: conf => !conf || (isArray(conf) ? conf : [conf]).every(conf => validatePluginConfig(conf)),
       message: