fix(opensmt/minsat): fix default argument in template friend function problem

Clang++-3.4 is starting to enforce the following item of C++11 standard,
thus it's making lean not compiling:

It's illegal in C++11: §8.3.6.4 [dcl.fct.default]
"If a friend declaration specifies a default argument expression, that
declaration shall be a definition and shall be the only declaration of
the function or function template in the translation unit."
feat(*): remove boost dependencies
feat(dsolvers/util/string): add starts_with, ends_with, join
feat(dsolvers/util/logger): add simple logger
chore(cmake): update dsolvers/util/util/CMakeLists.txt
feat(opensmt/smtsolvers): use logger instead of boost.log
feat(dsolvers): use logger in icp,nra,ode solvers
src(dsolvers, opensmt): use custom starts_with, ends_with, join methods
feat(tools/bmc): disable subgrouping
doc(*): remove boost
chore(travis): remove boost
refactor(dsolvers/util/logger): rename log functions to DREAL_LOG_*
doc(README.md): remove boost

[skip ci]
fix(*): remove remained boost dep.
fix(dsolvers/util/logger): fix log level
benchmark(hybrid_systems): update Makefile to point bmc.sh (rel-path)

[skip ci]
refactor(filib): remove filib
test(nra): add more regression tests (thanks to Fedor)
feat(cmake): use ExternalProject to build capd-dynsys and filib++
feat(realpaver/rp_interval): implement rp_interval_* funcs using filib++