Update jekyll version to latest used by GH pages

Hi, I'm new to Jekyll, so please tell me if I'm doing something wrong!

I forked your gh pages site, and installed ruby, and attempted to build a local version of the site using jekyll, as follows:

```
bundle install
jekyll build
```

It failed because it assumes the encoding of the pages was US-ASCII, when in fact they are UTF-8. For example:

```
$ jekyll build --trace
Configuration file: /Users/maloneyc/git/klortho/ncbi-dev-portal/_config.yml
            Source: /Users/maloneyc/git/klortho/ncbi-dev-portal
       Destination: /Users/maloneyc/git/klortho/ncbi-dev-portal/_site
      Generating... Error reading file /Users/maloneyc/git/klortho/ncbi-dev-portal/_posts/2013-08-21-environmental-toolkit.markdown: invalid byte sequence in US-ASCII
```

I found [this jekyll issue](https://github.com/jekyll/jekyll/issues/960) which describes the problem, and I noticed that it works correctly in later versions of jekyll.

So, I updated the Gemfile to specify the [current version of jekyll used by GH pages](https://pages.github.com/versions/), 2.4.0.  I took out the hard-coded version numbers of the other libraries, because I kept getting dependency conflicts, and I *think* that's what the Gemfile.lock does, is freeze the versions of the library dependencies.

This fixes the problem, and it now builds without a problem on my mac.  It also generates GH pages correctly, see my version: [http://klortho.github.io/ncbi-dev-portal/](http://klortho.github.io/ncbi-dev-portal/).

Hopefully, if you accept this pull request, it will save the next person some pain.  Plus, I think the jekyll version should be kept up-to-date with what's being used on GH pages.