Remove support for Julia 0.3 from src/test/readme of sorted containers

[StephenVavasis]

@simonster is preparing to drop Julia 0.3 support for DataStructures.jl. This PR remove references to 0.3 from the portions of src, readme and test files that pertain to sorted containers.

[StephenVavasis]

Note: I did not update the file src/DataStructures.jl because I didn't want to create a merge-conflict with another branch. When you are ready to drop 0.3 support, please edit src/DataStructures.jl also.

[StephenVavasis]

Another remark: I updated .travis.yml so that I could see 'All checks have passed' but I did not update REQUIRE. Someone also needs to update REQUIRE when 0.3 support is removed.

[hayd]

+1. IIUC this means the tests are going to fail on 0.3 so we should adjust the REQUIRE at the same time.

[kmsquire]

Closed by #155

[kmsquire]

(I included this commit in that PR, still attributed to @StephenVavasis.)

[StephenVavasis]

@kmsquire, just a quick note that there are still pre-0.4 references in src/DataStructures.jl if you want to thoroughly clean this up.

[kmsquire]

Thanks @StephenVavasis--there were actually quite a few more! See #156