Fix build for boost library from Ubuntu 13.04 repo

Building gearbox for Ubuntu 13.04 failed with linker error:
```
vagrant@vagrant gearbox$ make
make  all-am
make[1]: Entering directory `/home/vagrant/gearbox'
/bin/bash ./libtool --tag=CXX   --mode=link g++  -g -fpermissive
-L/usr/lib -lboost_program_options  -o workers/gearbox/workerGearbox
workers/gearbox/workerGearboxMain.o gearbox/core/libgearbox_core.la
gearbox/store/libgearbox_store.la workers/gearbox/libworker_gearbox.la
libtool: link: g++ -g -fpermissive -o
workers/gearbox/.libs/workerGearbox workers/gearbox/workerGearboxMain.o
-L/usr/lib -lboost_program_options gearbox/core/.libs/libgearbox_core.so
gearbox/store/.libs/libgearbox_store.so
workers/gearbox/.libs/libworker_gearbox.so
/home/vagrant/gearbox/gearbox/worker/.libs/libgearbox_worker.so
/home/vagrant/gearbox/gearbox/store/.libs/libgearbox_store.so
-lsoci_core /home/vagrant/gearbox/gearbox/job/.libs/libgearbox_job.so
-lgearman -lboost_graph_parallel
/home/vagrant/gearbox/gearbox/core/.libs/libgearbox_core.so -lyajl
/usr/lib/x86_64-linux-gnu/libcurl.so -lboost_filesystem -llog4cxx
-lboost_regex -lcrypto -luuid -lboost_iostreams
/usr/bin/ld: workers/gearbox/workerGearboxMain.o: undefined reference to
symbol '_ZN5boost6system15system_categoryEv'
/usr/bin/ld: note: '_ZN5boost6system15system_categoryEv' is defined in
DSO /usr/lib/libboost_system.so.1.53.0 so try adding it to the linker
command line
/usr/lib/libboost_system.so.1.53.0: could not read symbols: Invalid
operation
collect2: error: ld returned 1 exit status
make[1]: *** [workers/gearbox/workerGearbox] Error 1
make[1]: Leaving directory `/home/vagrant/gearbox'
make: *** [all] Error 2
```
Had to add -lboost_system explicitly to fix given error.