Update build for gcc 4.8.x:
* Use gcc 4.8.x, boost 1.54
* Honor CC,CXX,PATH environment variables
* Prevent compilation with unsupported toolchains
* Print g++ version during Travis build
* Run built-in unit tests after Travis build
[REMOVE] Allow gcc 4.7 and later
Remove files inadvertently added
PathRequests object to own all path requests.
SHAMapSync cleanups and performance improvements
Order book subscribe cleanups.
Add LES::dirIsEmpty function to check if a directory is empty
Reduce log priorities.
Add io_latency_probe
Limit legacy pathfinding requests
SHAMapSync bugfix. Cannot avoid push in getMissingNodes
Fix io_latency_probe hang on exit
Improved PropertyStream find and print routines
Don't send nodes twice in getNodeFat
Correctly handle empty account tree in ledger fetch
Dispatch peerHas to JobQueue
Workaround for MSVC std::function bug
Fix Clang compile and link errors
Add std::make_unique
Refactor KeyCache