Upgrade gcc in Travis for fixing issue with unmet dependencies while installing gcc-multilib.
Upgrade gcc in Travis for fixing issue with unmet dependencies while installing gcc-multilib.

cblichmann/yara,pombredanne/yara,pombredanne/yara,VirusTotal/yara,plusvic/yara,cblichmann/yara,pombredanne/yara,plusvic/yara,VirusTotal/yara,VirusTotal/yara,pombredanne/yara,VirusTotal/yara,hillu/yara,plusvic/yara,cblichmann/yara,VirusTotal/yara,plusvic/yara,wxsBSD/yara,wxsBSD/yara,hillu/yara,pombredanne/yara,hillu/yara,cblichmann/yara,wxsBSD/yara,hillu/yara,hillu/yara,cblichmann/yara,wxsBSD/yara,plusvic/yara,wxsBSD/yara