Switch to boost_1_61_0 name.

.travis.yml

@@ -202,7 +202,7 @@ script:
       cd "${TRAVIS_BUILD_DIR}/.."
       wget "https://raw.githubusercontent.com/boostorg/release-tools/develop/MakeBoostDistro.py"
       chmod +x MakeBoostDistro.py
-      export BOOST_RELEASE=boost_1_61_0_b1_rc2
+      export BOOST_RELEASE=boost_1_61_0
       ./MakeBoostDistro.py "${TRAVIS_BUILD_DIR}" "${BOOST_RELEASE}"
     fi