Node.js 4 support

[Igmat]

Working on issue #35 I've realized that our package doesn't support Node.js 4 (at least it doesn't correctly tested under this version). But Jest supports it, so we have to provide our users with such compatibility.

[Igmat]

I've changed our test cases a little bit. We can't target ES6 there, because node.js 4.x doesn't fully supports its syntax.
May be we have to add some more test-cases to check that it works and improve our docs with mentioning that our users can't use ES6 target while using node.js 4.x.
This probably will also lead to checking ts-jest working with babel-jest together.

But I don't know how to separate test cases for different node versions. @kulshekhar, any thoughts?
P.S.
I'm using nodist for local testing.

[kulshekhar]

The way I look at it, node 4.x will stop being maintained in about 5 months, So I don't see any point in supporting it.

[Igmat]

Due to LTS Plan v4 will be maintained until 2018-04-01, so I guess we have to support it unless Jest will stop doing this.

[kulshekhar]

In that case, it might be helpful to update the README with the status of support for various versions (the latest for 4 and 6 each?)

btw, this can get complicated very fast. I spent a few hours yesterday all because some tests were failing on 6.6 and on 6.3. If this is the case between 6.x versions, I shudder to think what's in store if we are going to support 4.x 😰

[Igmat]

Ok, build on travis was failed due to not latest npm package. Original Jest seems to have same issue on travis, so I've used their workaround.

I guess we have to show status for different versions of node and will try to find out how it could be accomplished with travis.

[kulshekhar]

@Igmat Is this good to publish? Maybe node 4 can be removed from the allowed failures list in travis.

[Igmat]

@kulshekhar, I've already published it. And I guess we could close this issue.
I've also added badges for different node versions, but I'm not sure that it works always correctly. So, please, if you find that it shows incorrect status feel free to post issue in my repo for badges.