travis: ensure COVERALLS_REPO_TOKEN is set before uploading
travis: ensure COVERALLS_REPO_TOKEN is set before uploading

PRs from 3rd party repos don't have access to encrypted env
variables.

For example, see:
https://travis-ci.org/intel/tpm2-tss/jobs/315268038

Signed-off-by: William Roberts <60148e894dabf0b83bde1d460768794cd453c455@intel.com>

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