Install "semver" package

It is required to implement the utility function to get Git version
information during build time.

References:
  https://semver.org
  https://www.npmjs.com/package/semver

GH-106

package.json

@@ -108,6 +108,7 @@
     "react-dom": "16.7.0",
     "react-helmet": "5.2.0",
     "react-pose": "4.0.4",
+    "semver": "5.6.0",
     "styled-components": "4.1.3",
     "styled-modern-normalize": ">=0.2.0 <1.0.0",
     "styled-theming": "2.2.0",