chore(travis): don't build dynamically linked Linux binaries for release

because musl binaries start up even quicker (2~3ms on a Broadwell Xeon,
compared to 4ms of glibc builds)

Author: xen0n

.travis.yml

@@ -31,7 +31,6 @@ matrix:
     - env: TARGET=aarch64-unknown-linux-gnu
     - env: TARGET=arm-unknown-linux-gnueabi
     - env: TARGET=armv7-unknown-linux-gnueabihf
-    - env: TARGET=i686-unknown-linux-gnu
     - env: TARGET=i686-unknown-linux-musl
     - env: TARGET=mips-unknown-linux-gnu
     - env: TARGET=mips64-unknown-linux-gnuabi64
@@ -41,7 +40,6 @@ matrix:
     - env: TARGET=powerpc64-unknown-linux-gnu
     - env: TARGET=powerpc64le-unknown-linux-gnu
     - env: TARGET=s390x-unknown-linux-gnu DISABLE_TESTS=1
-    - env: TARGET=x86_64-unknown-linux-gnu
     - env: TARGET=x86_64-unknown-linux-musl
 
     # *BSD