travis.yml: Use TRAVIS_BUILD_DIR to refer to the top directory

Travis already has an environment variable that points to the top of
the checked-out source code, TRAVIS_BUILD_DIR. We can use it to avoid
the guessing of the right location of the configure script.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20200925071147.149406-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>