[master] increase library version

Changed existing interface; see commit
219324e66e5bf33fde5cd53dc19f82fb1d442fc8 about the refactoring.

The error seen was:

[ RUN      ] PerfPkt4Test.RawUnpack
/libexec/ld-elf.so.1: /usr/home/jreed/builder/work/BIND10-cppcheck/20120620073005-FreeBSD8-amd64-GCC/build/tests/tools/perfdhcp/tests/.libs/run_unittests: Undefined symbol "_ZN3isc4dhcp6Option7setDataEN9__gnu_cxx17__normal_iteratorIPKhSt6vectorIhSaIhEEEES9_"

src/lib/dhcp/Makefile.am

@@ -36,7 +36,7 @@ libdhcp___la_CXXFLAGS = $(AM_CXXFLAGS)
 libdhcp___la_CPPFLAGS = $(AM_CPPFLAGS) $(LOG4CPLUS_INCLUDES)
 libdhcp___la_LIBADD   = $(top_builddir)/src/lib/asiolink/libasiolink.la
 libdhcp___la_LIBADD  += $(top_builddir)/src/lib/util/libutil.la
-libdhcp___la_LDFLAGS  = -no-undefined -version-info 1:0:0
+libdhcp___la_LDFLAGS  = -no-undefined -version-info 2:0:0
 
 if USE_CLANGPP
 # Disable unused parameter warning caused by some of the