feat(release): replace semantic-release dep with Travis build stages

This change moves semantic-release out of dev-dependencies, but keeps
its functionality by calling semantic-release as usual in a `release`
TravisCI build stage. There are several advantages to this method:

1.) semantic-release is run only after all builds succeed. Our previous
approach could have theoretically run semantic-release when some Node.js
versions failed with the current code
2.) semantic-release (and it's many dependencies) are removed from
`node_modules`. This increases the speed of `npm install` in all cases,
and reduces the size of our Docker images by 20MB (from 284MB to 264MB)!
Since the only time semantic-release is needed is on TravisCI anyway, it
seems pointless that every installation of Pelias should include it.
3.) Because semantic-release is not in `package.json`, Greenkeeper will
not attempt to update it. Semantic release updates _very_ frequently,
and each update attempt seems to have a decent chance of experiencing a
random TravisCI failure, causing unwanted notifications.

There are probably downsides to this approach. For example, we should
consider pinning the major version of semantic release during install.

Additionally, and for obvious reasons, we can't fully test this change
until it's merged to the `production` branch. We should consider testing
it first on a lower priority repository.

If this change _does_ work well, we should consider adopting it
everywhere.

.travis.yml

@@ -11,8 +11,11 @@ matrix:
 script: npm run travis
 before_install:
   - npm i -g npm
-after_success:
-  - npx semantic-release
 branches:
   except:
     - /^v\d+\.\d+\.\d+$/
+jobs:
+  include:
+    - stage: release
+      node_js: 10
+      script: npx semantic-release

package.json

@@ -75,7 +75,6 @@
     "pelias-mock-logger": "^1.3.0",
     "precommit-hook": "^3.0.0",
     "proxyquire": "^2.0.0",
-    "semantic-release": "^15.1.4",
     "source-map": "^0.7.0",
     "tap-dot": "^2.0.0",
     "tape": "^4.5.1",