Fix Travis CI build

- Install and use GNU GCC 4.8 to support required C++11 standard
- Build Boost (1.55.0) libraries needed by ledger
- Export build and cmake variables to find locally built Boost
Add URL to Travis CI environment documentation

[ci skip]
improve C-c C-a (ledger-xact-insert-transaction) behaviour

When the right place to add a transaction is after all existing
transactions, add it just after the last transaction rather than at the
end of the buffer.  (Otherwise the transactions get added after Local
Variables blocks and any other endmatter.)
Merge pull request #278 from afh/pull/travis

Fix Travis CI build
Merge pull request #279 from csrhodes/fix-ledger-xact-add-transaction

improve C-c C-a (ledger-xact-insert-transaction) behaviour