[CI] Use git shallow clone to speed up

.travis.yml

@@ -185,12 +185,12 @@ before_deploy:
 - mkdir -p /tmp/bin /tmp/lib /tmp/include
 - export LD_LIBRARY_PATH=/tmp/lib:LD_LIBRARY_PATH
 - export PATH=/tmp/bin:$PATH
-- git clone https://github.com/jedisct1/libsodium.git --branch=stable
+- git clone --dpeth 1 https://github.com/jedisct1/libsodium.git --branch=stable
 - cd libsodium
 - env ./configure --disable-dependency-tracking --prefix=/tmp
 - make -j$(nproc) install
 - cd -
-- git clone https://github.com/jedisct1/minisign.git
+- git clone --depth 1 https://github.com/jedisct1/minisign.git
 - cd minisign/src
 - gcc -O2 -o /tmp/bin/minisign -I/tmp/include -L/tmp/lib *.c -lsodium
 - cd -