Switch CI builds to use bundled JSON

Apparently the nlohmann library is not available in the standard
repositories.  I would rather build it than add universe deps.

.ci/install.sh

@@ -45,5 +45,4 @@ if [ "$TRAVIS_OS_NAME" == "linux" ]; then
         libqt5svg5-dev \
         qtmultimedia5-dev \
         liblmdb-dev \
-        nlohmann-json-dev
 fi

.travis.yml

@@ -31,7 +31,7 @@ matrix:
               - DEPLOYMENT=1
               - USE_BUNDLED_BOOST=1
               - USE_BUNDLED_CMARK=1
-              - USE_BUNDLED_JSON=0
+              - USE_BUNDLED_JSON=1
           addons:
               apt:
                   sources: ["ubuntu-toolchain-r-test"]
@@ -45,7 +45,7 @@ matrix:
               - QT_PKG=57
               - USE_BUNDLED_BOOST=1
               - USE_BUNDLED_CMARK=1
-              - USE_BUNDLED_JSON=0
+              - USE_BUNDLED_JSON=1
           addons:
               apt:
                   sources: ["ubuntu-toolchain-r-test"]
@@ -59,7 +59,7 @@ matrix:
               - QT_PKG=59
               - USE_BUNDLED_BOOST=1
               - USE_BUNDLED_CMARK=1
-              - USE_BUNDLED_JSON=0
+              - USE_BUNDLED_JSON=1
           addons:
               apt:
                   sources: ["ubuntu-toolchain-r-test", "llvm-toolchain-trusty-5.0"]

Dockerfile

@@ -11,7 +11,6 @@ RUN \
 
 RUN \
     apt-get install -y \
-        nlohmann-json-dev \
         make \
         pkg-config \
         ninja-build \