Change default b_ndebug value

The value of setting this to "if-release" seems dubious. Additionally,
this allows more recent versions of Ubuntu to work without installing a
newer Meson.

.travis.yml

@@ -20,7 +20,7 @@ before_install:
 
 script:
     - docker run --rm icecream-sundae-$TEST_OS /bin/sh -c "CC=$CC CXX=$CXX
-        meson -Dwerror=true -Db_sanitize=address,undefined .. &&
+        meson -Db_ndebug=false -Dwerror=true -Db_sanitize=address,undefined .. &&
         ninja &&
         ninja test &&
         ninja install"

.travis/ubuntu-rolling/Dockerfile

@@ -7,11 +7,8 @@ RUN apt-get -y update && apt-get -y install \
     libicecc-dev \
     libncursesw5-dev \
     meson \
-    python3-pip \
     ninja-build
 
-RUN pip3 install meson
-
 RUN mkdir -p /root/icecream-sundae/builddir
 COPY . /root/icecream-sundae/
 WORKDIR /root/icecream-sundae/builddir/

meson.build

@@ -1,10 +1,11 @@
 project('icecream-sundae', 'cpp',
         version: '0.99.0',
+        meson_version: '>= 0.31.0',
         default_options: [
             'buildtype=debugoptimized',
             'cpp_std=c++14',
             'warning_level=3',
-            'b_ndebug=if-release',
+            'b_ndebug=true',
             ])
 
 icecc = dependency('icecc')