Fix compilation on Debian 7 i686 due to 3 missing boost::uint64_t  casts in State.cc

Compilation was failing with:

Scanning dependencies of target btest
[ 83%] Building CXX object libgrive/CMakeFiles/btest.dir/test/btest/ValTest.cc.o
[ 85%] Building CXX object libgrive/CMakeFiles/btest.dir/test/btest/UnitTest.cc.o
[ 87%] Building CXX object libgrive/CMakeFiles/btest.dir/test/btest/JsonValTest.cc.o
Linking CXX executable btest
[ 87%] Built target btest
Scanning dependencies of target grive_executable
[ 89%] Building CXX object grive/CMakeFiles/grive_executable.dir/src/main.cc.o
Linking CXX executable grive
../libgrive/libgrive.a(State.cc.o): In function `gr::v1::State::Write(boost::filesystem3::path const&) const':
State.cc:(.text+0x1778): undefined reference to `gr::Json::Json<long>(long const&)'
State.cc:(.text+0x17fa): undefined reference to `gr::Json::Json<unsigned long>(unsigned long const&)'
State.cc:(.text+0x18cd): undefined reference to `gr::Json::Json<long>(long const&)'
collect2: error: ld returned 1 exit status
make[2]: *** [grive/grive] Error 1
make[1]: *** [grive/CMakeFiles/grive_executable.dir/all] Error 2
make: *** [all] Error 2


Host system is:
Linux debian 3.2.0-4-686-pae #1 SMP Debian 3.2.57-3+deb7u1 i686 GNU/Linux