Download the Hashcat utils
wget https://github.com/hashcat/hashcat-utils/archive/master.zip
Run the following commands to unzip and make the binaries
unzip master.zip cd hashcat-utils-master/src make
You can now convert an aircrack file by invoking the cap2hccapx binary
./cap2hccapx.bin /path/to/aircrack.cap /path/for/output