add travis yml

hi, I am one of the fan of this project :D
adding .travis.yml for *Automated Testing*.
unfortunately, test does not seem to pass.
if is it ok, would you please disable these tests?
```
test/alert_tests.cpp(126): error in "AlertApplies": check alert.CheckSignature() failed
test/alert_tests.cpp(126): error in "AlertApplies": check alert.CheckSignature() failed
test/alert_tests.cpp(126): error in "AlertApplies": check alert.CheckSignature() failed
test/alert_tests.cpp(126): error in "AlertApplies": check alert.CheckSignature() failed
test/alert_tests.cpp(126): error in "AlertApplies": check alert.CheckSignature() failed
test/alert_tests.cpp(126): error in "AlertApplies": check alert.CheckSignature() failed
test/alert_tests.cpp(126): error in "AlertApplies": check alert.CheckSignature() failed
test/alert_tests.cpp(126): error in "AlertApplies": check alert.CheckSignature() failed
test/alert_tests.cpp(129): error in "AlertApplies": check alerts[0].AppliesTo(1, "") failed
test/alert_tests.cpp(130): error in "AlertApplies": check alerts[0].AppliesTo(70001, "") failed
test/alert_tests.cpp(131): error in "AlertApplies": check alerts[0].AppliesTo(1, "/Satoshi:11.11.11/") failed
test/alert_tests.cpp(133): error in "AlertApplies": check alerts[1].AppliesTo(1, "/Satoshi:0.1.0/") failed
test/alert_tests.cpp(134): error in "AlertApplies": check alerts[1].AppliesTo(70001, "/Satoshi:0.1.0/") failed
test/alert_tests.cpp(136): error in "AlertApplies": check alerts[2].AppliesTo(1, "/Satoshi:0.1.0/") failed
test/alert_tests.cpp(137): error in "AlertApplies": check alerts[2].AppliesTo(1, "/Satoshi:0.2.0/") failed
test/alert_tests.cpp(173): error in "AlertNotify": check r.size() == 1u failed [0 != 1]
unknown location(0): fatal error in "AlertNotify": memory access violation at address: 0x00000000: no mapping at fault address
test/alert_tests.cpp(174): last checkpoint
```