doc(ubuntu-clang): add stdlibc++-4.8 requirement

[skip ci]
chore(cmake): make capd optional
doc(ubuntu-gcc): update apt-get options, fix opam parameter
doc(osx-gcc.md): update boost build instruction
fix(realpaver/rp_float.h): comment out extern math function decls
chore(cmake): update ODE-related message
fix(cmake): update EGLIBC setting
chore(travis): bump OSX compiler version
chore(travis): enable USE_LATEST_EGLIBC