Add ccache to speed up travis runs.
Add ccache to speed up travis runs.

Signed-off-by: Dominic Grauvogl <e4889dbaae35927834eb6dc5f1a4f9838882aacc@infineon.com>

01org/tpm2-tss,01org/TPM2.0-TSS,01org/TPM2.0-TSS,tpm2-software/tpm2-tss,01org/TPM2.0-TSS,01org/TPM2.0-TSS,01org/tpm2-tss,01org/tpm2-tss,tpm2-software/tpm2-tss,tpm2-software/tpm2-tss