Bump required node version to 0.12, the last supported LTS - #3181

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

Node.js >=0.6 is supported according to the package.json.

What is the new behavior?

Node.js >= 0.12 is supported. On Slack it was discussed that we should only support active LTS releases.

Does this PR introduce a breaking change?

• Yes
• No

If this PR contains a breaking change, please describe the following...

• Impact: If the user is running a Node.js version older then 0.12, webpack can break.
• Migration path for existing applications: Update Node.js to at least 0.12
• Github Issue(s) this is regarding:

Other information:

//cc @NekR @danez