v4.2.1

• add 'confirm_on_quit' option to world section to skip confirmation dialog on exit
• add continuous integration support by Travis CI
• fix segfault in ranger visualization
• fix segfault in blockgroup.cc:236 (gluTessEndPolygon()) when model size (x,y,z) is 0.0
• minor UI improvments (allow changing Stage's caption, allow centering the window)
• add ability to load worlds/models from std::istream for more flexibility
• fix compiler errors in debug build
• make odom integration error publicly accessible (see rtv#67)
• don't crash/segfault in ctrl/wander when there's only 1 range sensor (see rtv#59)
• fix errors reported by the most recent gcc 6.3.1 (on Debian 9 SID), rtv#63 in particular
• improve/unify coding style (strip trailing spaces, use clang-format)
• fix compiler error reported with strict flags (-Wall -Wextra -pedantic -Werror)
• fix issues reported by cppcheck v1.72
• minor updates in docs (cosmetics, make comments visible in doxygen)