Travis errors

[amatsuo]

As you can see from the tragic history of build of readtext, the package is now failing for Travis.

The problem is the readtext's dependency of streamR, which we use for the stream-based reading of raw JSON of Tweets in a text file where each line is one Tweet. This function is working flawlessly and efficiently in practical use. However, the compilation of ndjson package fails on Travis because the original C++ code cannot be compiled with the older version of gcc/g++ (see README of ndjson in R).

I have tried different combinations of gcc/g++ versions but have not been successful. So I just open an issue here to get some insights from other developers. The Travis build has been failing for many tries, and has never been resolved.

At one point, I was able to avoid this problem by forcing gcc-5 and g++-5 as the compiler. Somehow it worked in the past, but not any more.

Some notes:

• The difficult part of this problem is we cannot rely on either streamR or ndjson for solution:
  - streamR does not bother getting the travis badge (https://github.com/pablobarbera/streamR)
  - ndjson has tried to resolve (https://github.com/hrbrmstr/ndjson) but has not been successful (https://travis-ci.org/hrbrmstr/ndjson/builds).
• One of the issues is that Travis is still using Ubuntu14.04 (see a blog post by @eddelbuettel). It's not so easy to use the newer version of gcc/g++ without breaking compilation of other packages.