Fix a broken travis build (missing pthread library when linking a test).
Fix a broken travis build (missing pthread library when linking a test).

ocelot-inc/tarantool,Sannis/tarantool,nvoron23/tarantool,vasilenkomike/tarantool,condor-the-bird/tarantool,vasilenkomike/tarantool,rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool,mejedi/tarantool,vasilenkomike/tarantool,Sannis/tarantool,Sannis/tarantool,nvoron23/tarantool,KlonD90/tarantool,dkorolev/tarantool,dkorolev/tarantool,guard163/tarantool,KlonD90/tarantool,guard163/tarantool,guard163/tarantool,rtsisyk/tarantool,dkorolev/tarantool,vasilenkomike/tarantool,guard163/tarantool,nvoron23/tarantool,dkorolev/tarantool,mejedi/tarantool,Sannis/tarantool,KlonD90/tarantool,KlonD90/tarantool,nvoron23/tarantool,KlonD90/tarantool,condor-the-bird/tarantool,dkorolev/tarantool,condor-the-bird/tarantool,ocelot-inc/tarantool,rtsisyk/tarantool,guard163/tarantool,ocelot-inc/tarantool,ocelot-inc/tarantool,Sannis/tarantool,vasilenkomike/tarantool,nvoron23/tarantool,condor-the-bird/tarantool,condor-the-bird/tarantool,rtsisyk/tarantool,nvoron23/tarantool